Prior art thin-film amorphous silicon (a-Si) photovoltaic cells are built on a glass substrate as a series of layers. When the finished cell is operating, the glass substrate becomes a “superstrate”—the top layer through which the sunlight enters. Some of a solar cell's film layers are active (they participate in the actual conversion of light to electricity) and some are passive (they protect the active layers or the substrate). The first active layer deposited (closest to the glass) is usually a transparent conductive layer, such as tin oxide (SnO2). SnO2 is a transparent conductor that conducts current and voltage from the silicon PIN diode deposited thereabove (but located therebelow during operation) that converts sunlight into electricity. Because it is transparent, unlike ordinary conductors like aluminum, the SnO2 layer allows the sunlight to travel from the glass into the silicon.
In addition to its ability to transmit light and conduct electricity, SnO2 also has several other key advantages. It has good adhesion to glass. It also is very resistant to degradation by chemical attack in humid environments. In addition, SnO2 has the right value of work function to make a good contact to the p-layer of the silicon PIN diode that converts the sunlight to electricity. For soda-lime glass of the type typically used in the fabrication of glass superstrates for amorphous silicon photovoltaic cells, the adhesion, electrical properties and optical properties are often improved by coating the soda-lime glass with a very thin layer of SiO2 or SiOxCy that acts as a barrier to keep sodium from the glass away from the SnO2.
However, SnO2 has several disadvantages. First, its resistance is higher and its transparency is not as good as several other well known transparent conductors not used in amorphous silicon photovoltaic cells, such as ZnO. ZnO is inexpensive and abundant, but has several serious disadvantages as compared to SnO2. ZnO tends to be hygroscopic. It readily absorbs water and is easily attacked chemically in a humid environment. When applied to the surface of glass, for a variety of reasons, it adheres poorly and readily peels off the surface of the glass. In addition, ZnO is a soft material that is easily scratched and damaged mechanically.
ZnO has been demonstrated as a possible front conductor for a-Si photovoltaic cells in the laboratory. Tests have shown that the higher transparency and lower resistivity of ZnO produce, in a laboratory setting, photovoltaic cells with higher efficiency. However, the prior art fails to disclose the development of a production process for a-Si photovoltaic cells for use in the field with ZnO as the front conductor that can pass the usual reliability tests without the ZnO front conductor failing.
The present invention is directed to such a cell and a method therefor.